1. Field of the Invention
The present invention relates to a cellulose ester film, a retardation film, a polarizing plate and a liquid crystal display device.
2. Description of the Related Art
In silver halide photographic materials, retardation films, polarizing plates and image display devices, films of polymers represented by cellulose esters, polyesters, polycarbonates, cycloolefin polymers, vinyl polymers and polyimides and so on are used. Since films which are more excellent from the standpoints of planarity and uniformity can be manufactured from these polymers, they are widely employed as a film for optical applications. For example, cellulose ester films having adequate water vapor permeability can be stuck directly with a polarizer composed of the most general polyvinyl alcohol (PVA)/iodine through online. For that reason, cellulose esters, especially cellulose acetate, are widely employed as a protective film of polarizing plate.
In the case where a transparent polymer film is used for optical applications such as a retardation film, a support of retardation film, a protective film of polarizing plate and a liquid crystal display device, it is a very important factor to control the optical anisotropy in determining the performance of a display device (for example, visibility).
On the other hand, a solution fabrication process is widely utilized as a method for manufacturing a cellulose ester film to be used for optical applications. In that case, for the purpose of imparting high-speed fabrication adaptability during the manufacture, it is preferred to add a plasticizer. This is because by adding a plasticizer, it is possible to volatilize the solvent within a short period of time during drying at the time of solution fabrication. However, in a transparent polymer film containing a plasticizer which is usually used, there is, for example, the case where, when the polymer film is treated at a high temperature in a drying step, etc., smoking is generated; failures in operations are caused due to the attachment of a volatilized oil or the like to a manufacturing machine; or area defects are generated due to the attachment of stains to the polymer film. For that reason, there were naturally restrictions in the manufacture condition or treatment condition relative to the transparent polymer film using a plasticizer.
There is disclosed a technology of adding, as a polymeric plasticizer, a polyester and a polyester ether each having a mass average molecular weight of from 400 to 5,000 (see, for example, JP-A-2007-3767). According to this technology, though it is described that the technology is excellent in prevention of raw material deposition, water vapor permeability and dimension, it was insufficient in process contamination at the time of manufacture and raw material volatilization properties at the time of a stretching treatment. Furthermore, there is disclosed a cellulose ester film containing a polyester having an aromatic ring (see, for example, JP-A-2006-064803 and U.S. Pat. No. 5,559,171). However, even such a compound was not satisfactory from the standpoints of failures of operations of manufacturing equipment due to volatilization of the foregoing plasticizer, generation of area defects of film and performance with time in a form of polarizing plate.
On the other hand, in liquid crystal display devices, the use of an optically compensatory film for the purposes of enlarging a viewing angle, improving image coloration and enhancing the contrast is a widely known technology. In a most diffused VA (vertically aligned) mode or TN mode or the like, optically compensatory films capable of controlling particularly optical characteristics (for example, an Re value and an Rth value) to desired values are being demanded.
For the adjustment of optical characteristics suitable for the VA mode, the stretching treatment is required. A countermeasure against area defects to be caused due to the process contamination at the time of manufacture was strongly demanded.